Vertical transistor with enhanced drive current

ABSTRACT

A stacked vertical field effect transistor that has enhanced drive current is provided. The stacked vertical field effect transistor includes a lower functional gate structure located adjacent sidewall surfaces of a lower channel portion of a semiconductor channel material pillar. An upper functional gate structure is located above the lower functional gate structure and adjacent sidewall surfaces of an upper channel portion of the semiconductor channel material pillar. A bottom source/drain region is located beneath the lower functional gate structure, a middle source/drain region is located between the lower functional gate structure and the upper functional gate structure, and a top source/drain region is located above the upper functional gate structure.

BACKGROUND

The present application relates to a semiconductor structure and amethod of forming the same. More particularly, the present applicationrelates to a stacked vertical field effect transistor that has enhanceddrive current and a method of forming such a stacked vertical fieldeffect transistor.

Conventional vertical transistors are devices where the source-draincurrent flows in a direction normal to the substrate surface. In suchdevices, a vertical semiconductor pillar (or fin) defines the channelwith the source and drain located at opposing ends of the semiconductorpillar.

Vertical transistors are an attractive option for technology scaling.One potential drawback with conventional vertical transistors is thatdrive current for a given chip area is limited. As such, there is a needfor providing a vertical transistor in which the drive current isenhanced for a given chip area.

SUMMARY

The present application provides a semiconductor structure including astacked vertical field effect transistor that has enhanced drive currentand a method of forming such a stacked vertical field effect transistor.

In one aspect of the present application, a semiconductor structure isprovided. In one embodiment, the semiconductor structure includes alower functional gate structure located adjacent sidewall surfaces of alower channel portion of a semiconductor channel material pillar. Anupper functional gate structure is located above the lower functionalgate structure and adjacent sidewall surfaces of an upper channelportion of the semiconductor channel material pillar. A bottomsource/drain region is located beneath the lower functional gatestructure, a middle source/drain region is located between the lowerfunctional gate structure and the upper functional gate structure, and atop source/drain region is located above the upper functional gatestructure.

In another aspect of the present application, a method of forming asemiconductor structure is provided. In one embodiment, the methodincludes forming a material stack adjacent sidewall surfaces of asemiconductor channel material pillar. The material stack includes, frombottom to top, a bottom source/drain layer, a bottom spacer layer, afirst sacrificial gate structure, a first middle spacer layer, asacrificial spacer layer, a second middle spacer layer, a secondsacrificial gate structure and a top spacer layer. Next, the sacrificialspacer layer located between the first and second middle spacer layersis removed to physically expose sidewall surfaces of a middle portion ofthe semiconductor channel material pillar. A first epitaxial dopedsemiconductor material layer is epitaxially grown from exposed sidewallsurfaces of the middle portion of the semiconductor channel materialpillar, and a second epitaxial doped semiconductor material layer isalso epitaxially grown from exposed sidewall surfaces of an upperportion of the semiconductor channel material pillar. An anneal is thenperformed to diffuse dopants from the bottom source/drain layer into abottom portion of the semiconductor channel material pillar and toprovide a bottom source/drain region, dopants from the first epitaxialdoped semiconductor material layer into the middle portion of thesemiconductor channel material pillar and to provide a middlesource/drain region, and dopants from the second epitaxial dopedsemiconductor material layer into the upper portion of the semiconductorchannel material and to provide a top source/drain region. Next, thefirst sacrificial gate structure is replaced with a lower functionalgate structure, and the second sacrificial gate structure is replacedwith an upper functional gate structure.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a cross sectional view of an exemplary semiconductor structureincluding a semiconductor channel material pillar extending upward froma semiconductor material surface of a semiconductor substrate, andincluding a first material stack located adjacent sidewall surfaces ofthe semiconductor channel material pillar, the first material stackincludes a bottom source/drain layer, a bottom spacer layer, a firstsacrificial gate structure and a first middle spacer layer.

FIG. 2 is a cross sectional view of the exemplary semiconductorstructure of FIG. 1 after forming a second material stack adjacent thesidewall surfaces of the semiconductor channel material pillar and onthe first material stack, the second material stack including asacrificial spacer layer, a second middle spacer layer, a secondsacrificial gate structure and a top spacer layer.

FIG. 3 is a cross sectional view of the exemplary semiconductorstructure of FIG. 2 after removing the sacrificial spacer layer locatedbetween the first and second middle spacer layers to physically exposesidewall surfaces of a middle portion of the semiconductor channelmaterial pillar.

FIG. 4 is a cross sectional view of the exemplary semiconductorstructure of FIG. 3 after epitaxially growing a first epitaxial dopedsemiconductor material layer from exposed sidewall surfaces of themiddle portion of the semiconductor channel material pillar, and asecond epitaxial doped semiconductor material layer from exposedsidewall surfaces of an upper portion of the semiconductor channelmaterial pillar.

FIG. 5 is a cross sectional view of the exemplary semiconductorstructure of FIG. 4 after performing a drive in anneal to provide abottom source/drain region, a middle source/drain region and a topsource/drain region.

FIG. 6 is a cross sectional view of the exemplary semiconductorstructure of FIG. 5 after replacing the first sacrificial gate structurewith a lower functional gate structure, and the second sacrificial gatestructure with an upper functional gate structure.

FIG. 7 is a pictorial representation of the exemplary semiconductorstructure of the present application after forming various contactstructures.

DETAILED DESCRIPTION

The present application will now be described in greater detail byreferring to the following discussion and drawings that accompany thepresent application. It is noted that the drawings of the presentapplication are provided for illustrative purposes only and, as such,the drawings are not drawn to scale. It is also noted that like andcorresponding elements are referred to by like reference numerals.

In the following description, numerous specific details are set forth,such as particular structures, components, materials, dimensions,processing steps and techniques, in order to provide an understanding ofthe various embodiments of the present application. However, it will beappreciated by one of ordinary skill in the art that the variousembodiments of the present application may be practiced without thesespecific details. In other instances, well-known structures orprocessing steps have not been described in detail in order to avoidobscuring the present application.

It will be understood that when an element as a layer, region orsubstrate is referred to as being “on” or “over” another element, it canbe directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “beneath” or “under” another element, it can bedirectly beneath or under the other element, or intervening elements maybe present. In contrast, when an element is referred to as being“directly beneath” or “directly under” another element, there are nointervening elements present.

Referring first to FIG. 1, there is illustrated an exemplarysemiconductor structure that can be employed in accordance with anembodiment of the present application. The semiconductor structure ofFIG. 1 includes a semiconductor channel material pillar (or fin) 12extending upward from a semiconductor material surface of asemiconductor substrate 10. Although the present application isdescribed and illustrated with a single semiconductor channel materialpillar 12, the present application can be used in instances in which aplurality of spaced apart semiconductor channel material pillars isformed. In some embodiments, a hard mask cap 14 may be present on top ofthe semiconductor channel material pillar (or fin) 12.

The semiconductor structure of FIG. 1 further includes a first materialstack (16, 18, 20, 22) located adjacent sidewall surfaces of thesemiconductor channel material pillar 12 and on the semiconductorsubstrate 10. The first material stack includes a bottom source/drainlayer 16, a bottom spacer layer 18, a first sacrificial gate structure20 and a first middle spacer layer 22.

The semiconductor structure of FIG. 1 can be formed utilizingconventional techniques well known to those skilled in the art. In oneembodiment of the present application, the semiconductor structure ofFIG. 1 can be formed by first providing the semiconductor substrate 10.The semiconductor substrate 10 may include at least one semiconductormaterial having semiconducting properties. Examples of semiconductormaterials that may provide at least a portion of the semiconductorsubstrate 10 may include silicon (Si), germanium (Ge), silicon germaniumalloys (SiGe), silicon carbide (SiC), silicon germanium carbide (SiGeC),III-V compound semiconductors or II-VI compound semiconductors. III-Vcompound semiconductors are materials that include at least one elementfrom Group III of the Periodic Table of Elements and at least oneelement from Group V of the Periodic Table of Elements. II-VI compoundsemiconductors are materials that include at least one element fromGroup II of the Periodic Table of Elements and at least one element fromGroup VI of the Periodic Table of Elements.

In one embodiment, the semiconductor substrate 10 is a bulksemiconductor substrate. By “bulk” it is meant that the semiconductorsubstrate 10 is entirely composed of at least one semiconductormaterial, as defined above. In one example, the semiconductor substrate10 may be entirely composed of silicon. In some embodiments, the bulksemiconductor substrate may include a multilayered semiconductormaterial stack including at least two different semiconductor materials,as defined above. In one example, the multilayered semiconductormaterial stack may comprise, in any order, a stack of Si and a silicongermanium alloy.

In another embodiment of the present application, the semiconductorsubstrate 10 comprises a topmost semiconductor material layer of asemiconductor-on-insulator (SO) substrate. The SOI substrate would alsoinclude a handle substrate (not shown) including one of the abovementioned semiconductor materials, and an insulator layer (not shown)such as a buried oxide below the topmost semiconductor material layer.

In any of the above embodiments mentioned above, the semiconductormaterial that provides the semiconductor substrate 10 may be a singlecrystalline semiconductor material. The semiconductor material thatprovides the semiconductor substrate 10 may have any of the well knowncrystal orientations. For example, the crystal orientation of the basesemiconductor substrate 12 may be {100}, {110}, or {111}. Othercrystallographic orientations besides those specifically mentioned canalso be used in the present application.

After providing the semiconductor substrate 10, the semiconductorchannel material pillar 12 is formed. In one embodiment of the presentapplication, the semiconductor channel material pillar 12 can be formedby first providing a sacrificial dielectric material layer (not shown)on the entirety of the semiconductor material surface of thesemiconductor substrate 10. In one embodiment, the sacrificialdielectric material layer may include, for example, silicon dioxide. Thesacrificial dielectric material layer can be formed utilizing adeposition process such as, for example, chemical vapor deposition(CVD), plasma enhanced chemical vapor deposition, or physical vapordeposition.

Next, an opening is formed into the sacrificial dielectric materiallayer that exposes a portion of a semiconductor material surface of thesemiconductor substrate 10. The opening can be formed utilizinglithography and etching. The opening has a width and height that is usedin defining the width and height of the semiconductor channel materialpillar 12. In one embodiment, the opening that is formed into thesacrificial dielectric material layer has a width from 6 nm to 12 nm,and a height from 50 nm to 250 nm. The opening can have other widths andheights besides the ranges mentioned herein.

The semiconductor channel material pillar 12 is then epitaxially grown(or deposited) from the exposed portion of the semiconductor materialsurface of the semiconductor substrate 10. The terms “epitaxiallygrowing and/or depositing” and “epitaxially grown and/or deposited” meanthe growth of a semiconductor material on a deposition surface of asemiconductor material, in which the semiconductor material being grownhas the same crystalline characteristics as the semiconductor materialof the deposition surface. In an epitaxial deposition process, thechemical reactants provided by the source gases are controlled and thesystem parameters are set so that the depositing atoms arrive at thedeposition surface of the semiconductor substrate with sufficient energyto move around on the surface and orient themselves to the crystalarrangement of the atoms of the deposition surface. Therefore, anepitaxial semiconductor material has the same crystallinecharacteristics as the deposition surface on which it is formed. Sincean epitaxial growth process is used in providing the semiconductorchannel material pillar 12, the semiconductor channel material pillar 12has an epitaxial relationship with the exposed semiconductor materialsurface of the semiconductor substrate 10.

Examples of various epitaxial growth process apparatuses that can beemployed in the present application include, e.g., rapid thermalchemical vapor deposition (RTCVD), low-energy plasma deposition (LEPD),ultra-high vacuum chemical vapor deposition (UHVCVD), atmosphericpressure chemical vapor deposition (APCVD) and molecular beam epitaxy(MBE). The temperature for epitaxial deposition typically ranges from500° C. to 900° C. Although higher temperature typically results infaster deposition, the faster deposition may result in crystal defectsand film cracking. The epitaxial growth of the semiconductor channelmaterial pillar 12 can be performed utilizing any well known precursorgas or gas mixture. Carrier gases like hydrogen, nitrogen, helium andargon can be used. Typically, the semiconductor channel material pillar12 that is epitaxially grown is non-doped.

After forming the semiconductor channel material pillar 12, a recessetch may be performed to recess the height of the originally formedsemiconductor channel material pillar 12 and thereafter hard mask cap 14may be formed on the recessed semiconductor channel material pillar 12.The hard mask cap 14 may be composed of a hard mask material such as,for example, silicon dioxide, silicon nitride or silicon oxynitride; inthis embodiment, the hard mask material that provides the hard mask cap14 is composed of a different dielectric material than the sacrificialdielectric material layer. The hard mask material that provides the hardmask cap 14 may be formed utilizing a deposition process such as, forexample, chemical vapor deposition or plasma enhanced chemical vapordeposition. A planarization process may follow the deposition of thehard mask material.

After forming the hard mask capped semiconductor channel material pillar(12, 14), the remaining sacrificial dielectric material layer is removedutilizing an etching process that is selective in removing thedielectric material that provides the sacrificial dielectric materiallayer.

In another embodiment, the hard mask capped semiconductor channelmaterial pillar (12, 14) may be formed by epitaxially growing asemiconductor material that provides the semiconductor channel materialpillar 12 on the entirety of the semiconductor substrate 10, andthereafter forming a hard mask material on the epitaxially grownsemiconductor material. The stack of the semiconductor material and thehard mask material can then be patterned to provide the hard mask cappedsemiconductor channel material pillar (12, 14). Patterning may include,for example, lithography and etching, or a sidewall image transferprocess.

The semiconductor channel material pillar 12 may include one of thesemiconductor materials mentioned above for the semiconductor substrate10. In one embodiment, the semiconductor channel material pillar 12 iscomposed of a same semiconductor material as the semiconductor substrate10. In another embodiment, the semiconductor channel material pillar 12is composed of a different semiconductor material than the semiconductorsubstrate 10. In one embodiment, the semiconductor channel materialpillar 12 is composed of a semiconductor material that has a higherelectron mobility than the semiconductor material that provides thesemiconductor substrate 10. In such an embodiment, the semiconductorchannel material pillar 12 may be composed of a III-V compoundsemiconductor material, while the semiconductor substrate 10 may becomposed of silicon.

Next, the first material stack (16, 18, 20, 22) is formed adjacentsidewall surfaces of the semiconductor channel material pillar 12 and onthe semiconductor substrate 10. As mentioned above, the first materialstack includes bottom source/drain layer 16, bottom spacer layer 18,first sacrificial gate structure 20 and first portion of a middle spacerlayer 22.

The bottom source/drain layer 16 includes a semiconductor material and adopant (stated in other terms the bottom source/drain layer 16 iscomposed of a first doped semiconductor material). The semiconductormaterial that provides the bottom source/drain layer 16 may include oneof the semiconductor materials mentioned above for the semiconductorsubstrate 10. In one embodiment, the bottom source/drain layer 16 iscomposed of a same semiconductor material as the semiconductor substrate10 and the semiconductor channel material pillar 12. In anotherembodiment, the bottom source/drain layer 16 is composed of a differentsemiconductor material than the semiconductor substrate 10 and/or thesemiconductor channel material pillar 12. The dopant that is present inthe semiconductor material that provides bottom source/drain layer 16may be a p-type dopant or an n-type dopant. The term “p-type” refers tothe addition of impurities to an intrinsic semiconductor that createsdeficiencies of valence electrons. In a silicon-containing semiconductormaterial, examples of p-type dopants, i.e., impurities, include, but arenot limited to, boron, aluminum, gallium and indium. “N-type” refers tothe addition of impurities that contributes free electrons to anintrinsic semiconductor. In a silicon containing semiconductor material,examples of n-type dopants, i.e., impurities, include, but are notlimited to, antimony, arsenic and phosphorous.

In one embodiment, the concentration of n-type or p-type dopant withinthe semiconductor material that provides the bottom source/drain layer16 can range from 1×10¹⁸ atoms/cm³ to 1×10²¹ atoms/cm³, although dopantconcentrations greater than 1×10²¹ atoms/cm³ or less than 1×10¹⁸atoms/cm³ are also conceived. In one embodiment, the doping within thebottom source/drain layer 16 may be uniform (i.e., have a uniformdistribution of dopants throughout the entire region). In anotherembodiment, the doping within the bottom source/drain layer 16 may begraded.

The bottom source/drain layer 16 may be formed utilizing an epitaxialgrowth process as mentioned above. In some instances, an etch backprocess may follow the epitaxial growth of the bottom source/drain layer16. In some embodiments, the dopant that is present in the bottomsource/drain layer 16 may be introduced in-situ into the precursor gasor gas mixture that provides the bottom source/drain layer 16. Inanother embodiment, the dopant may be introduced into an intrinsicsemiconductor material by ion implantation or gas phase doping.

In one embodiment of the present application, the bottom source/drainlayer 16 may have a thickness from 10 nm to 50 nm. Other thicknessesthat are lesser than, or greater than, the aforementioned thicknessrange may also be employed in the present application as the thicknessof the bottom source/drain layer 16.

After forming the bottom source/drain layer 16, bottom spacer layer 18is formed on the physically exposed topmost surface of the bottomsource/drain layer 16. The bottom spacer layer 18 may be composed of anydielectric spacer material including for example, silicon dioxide,silicon nitride or silicon oxynitride.

The bottom spacer layer 18 may be formed utilizing a directionaldeposition process such as, for example, chemical vapor deposition orplasma enhanced chemical vapor deposition. In some instances, an etchback process may follow the deposition of the dielectric spacer materialthat provides the bottom spacer layer 18. The bottom spacer layer 18 mayhave a thickness from 4 nm to 10 nm. Other thicknesses that are lesserthan, or greater than, the aforementioned thickness range may also beemployed in the present application as the thickness of the bottomspacer layer 18.

After forming the bottom spacer layer 18, the first sacrificial gatestructure 20 is formed on the physically exposed topmost surface of thebottom spacer layer 18. The first sacrificial gate structure 20 may beformed utilizing a directional deposition process such as, for example,chemical vapor deposition or plasma enhanced chemical vapor deposition.In some instances, an etch back process may follow the deposition of thematerial that provides the first sacrificial gate structure 20. Thefirst sacrificial gate structure 20 may have a thickness from 10 nm to30 nm. Other thicknesses that are lesser than, or greater than, theaforementioned thickness range may also be employed in the presentapplication as the thickness of the first sacrificial gate structure 20.The first sacrificial gate structure 20 may be composed of polysilicon,amorphous silicon or any other material that can be used as aplaceholder material for a functional gate structure.

Next, the first middle spacer layer 22 is formed on the physicallyexposed topmost surface of the first sacrificial gate structure 20. Thefirst middle spacer layer 22 may be composed of one of the dielectricspacer materials mentioned above for the bottom spacer layer 18. In oneembodiment, the first middle spacer layer 22 is composed of a samedielectric spacer material as the bottom spacer layer 18. For example,the first middle spacer layer 22 and the bottom spacer layer 18 may becomposed of silicon nitride. In another embodiment, the first middlespacer layer 22 is composed of a different dielectric spacer materialthan the bottom spacer layer 18. For example, the first middle spacerlayer 22 may be composed of silicon nitride and the bottom spacer layermay be composed of silicon oxynitride.

The first middle spacer layer 22 may be formed utilizing a directionaldeposition process such as, for example, chemical vapor deposition orplasma enhanced chemical vapor deposition. In some instances, an etchback process may follow the deposition of the dielectric spacer materialthat provides the first middle spacer layer 22. The first middle spacerlayer 22 may have a thickness from 4 nm to 10 nm. Other thicknesses thatare lesser than, or greater than, the aforementioned thickness range mayalso be employed in the present application as the thickness of thefirst middle spacer layer 22.

Referring now to FIG. 2, there is illustrated the exemplarysemiconductor structure of FIG. 1 forming a second material stack (24,26, 28, 30) adjacent sidewall surfaces of the semiconductor channelmaterial pillar 12 and on the first material stack (16, 18, 20, 22). Thesecond material stack includes, from bottom to top, a sacrificial spacerlayer 24, a second middle spacer layer 26, a second sacrificial gatestructure 28 and a top spacer layer 30.

Collectively, the first material stack and the second material stack maybe referred to herein as merely a material stack. As is shown, thesecond material stack (and thus the combined first and second materialstacks) has a topmost surface that is located beneath a topmost surfaceof the semiconductor channel material pillar 12 such that upper sidewallsurfaces of the semiconductor channel material pillar 12 are physicallyexposed. It is noted that the material stack including the first andsecond material stacks laterally surround the semiconductor channelmaterial pillar 12.

The sacrificial spacer layer 24 is formed on the physically exposedtopmost surface of the first middle spacer layer 22. The sacrificialspacer layer 24 is composed of a different dielectric spacer materialthan the first middle spacer layer 22 and, if present, the hard mask cap14. For example, the first middle spacer layer 22 and, if present, thehard mask cap 14 may be composed of silicon nitride, while thesacrificial spacer layer 24 is composed of silicon dioxide. Thesacrificial spacer layer 24 may be formed utilizing a directionaldeposition process such as, for example, chemical vapor deposition orplasma enhanced chemical vapor deposition. In some instances, an etchback process may follow the deposition of the dielectric spacer materialthat provides the sacrificial spacer layer 24. The sacrificial spacerlayer 24 may have a thickness from 20 nm to 60 nm. Other thicknessesthat are lesser than, or greater than, the aforementioned thicknessrange may also be employed in the present application as the thicknessof the sacrificial spacer layer 24.

Next, the second middle spacer layer 26 is formed on the physicallyexposed topmost surface of the sacrificial spacer layer 24. The secondmiddle spacer layer 26 may be composed a same or different dielectricspacer material than the first middle spacer layer 22 provided that thedielectric spacer materials that provide the first and second middlespacer layers 22, 26 are both different from the dielectric spacermaterial that provides the sacrificial spacer layer 24. In one example,the first and second middle spacer layers 22, 26 are both composed of asame dielectric spacer material such as, for example, silicon nitride,while the sacrificial spacer layer 24 is composed of silicon dioxide.

The second middle spacer layer 26 may be formed utilizing a directionaldeposition process such as, for example, chemical vapor deposition orplasma enhanced chemical vapor deposition. In some instances, an etchback process may follow the deposition of the dielectric spacer materialthat provides the second middle spacer layer 26. The second middlespacer layer 26 may have a thickness from 4 nm to 10 nm. Otherthicknesses that are lesser than, or greater than, the aforementionedthickness range may also be employed in the present application as thethickness of the second middle spacer layer 26.

Next, the second sacrificial gate structure 28 is formed on thephysically exposed topmost surface of the second middle spacer layer 26.The second sacrificial gate structure 28 may be formed utilizing adirectional deposition process such as, for example, chemical vapordeposition or plasma enhanced chemical vapor deposition. In someinstances, an etch back process may follow the deposition of thematerial that provides the second sacrificial gate structure 28. Thesecond sacrificial gate structure 28 may have a thickness from 10 nm to30 nm. Other thicknesses that are lesser than, or greater than, theaforementioned thickness range may also be employed in the presentapplication as the thickness of the second sacrificial gate structure28. The second sacrificial gate structure 28 may be composed ofpolysilicon, amorphous silicon or any other material that can be used asa placeholder material for a functional gate structure. The first andsecond sacrificial gate structures 20, 28 are typically composed of asame material such that they can be removed together in a single step.

Next, the top spacer layer 30 is formed on the physically exposedtopmost surface of the second sacrificial gate structure 28. The topspacer layer 30 may be composed of one of the dielectric spacermaterials mentioned above for the bottom spacer layer 18; the top spacerlayer 30 must be composed of a different spacer dielectric material thanthe sacrificial spacer layer 24. In one embodiment, the top spacer layer30 is composed of a same dielectric spacer material as the bottom spacerlayer 18 and optionally the first and second middle spacer layers 22,26. For example, the top spacer layer 30, the bottom spacer layer 18 andthe first and second middle spacer layers 22, 24 may each be composed ofsilicon nitride.

The top spacer layer 30 may be formed utilizing a directional depositionprocess such as, for example, chemical vapor deposition or plasmaenhanced chemical vapor deposition. In some instances, an etch backprocess may follow the deposition of the dielectric spacer material thatprovides the top spacer layer 30. The top spacer layer 30 may have athickness from 4 nm to 10 nm. Other thicknesses that are lesser than, orgreater than, the aforementioned thickness range may also be employed inthe present application as the thickness of the top spacer layer 30.

Referring now to FIG. 3, there is illustrated the exemplarysemiconductor structure of FIG. 2 after removing the sacrificial spacerlayer 24 located between the first and second middle spacer layers 22,26 to physically expose sidewall surfaces of a middle portion of thesemiconductor channel material pillar 12. The sacrificial spacer layer24 may be removed utilizing an etching process that is selective inremoving the dielectric spacer material of the sacrificial spacer layer24 relative to the dielectric spacer materials that provide the topspacer layer 30, the first and second middle spacer layers 22, 26 and,if present, the hard mask cap 14. In one embodiment, and when thedielectric spacer material that provides the sacrificial spacer layer 24is composed of silicon dioxide, and the dielectric spacer material thatprovides each of the top spacer layer 30, the first and second middlespacer layers 22, 26 and, if present, the hard mask cap 14 is composedof silicon nitride, hydrofluoric acid or a buffered etch (i.e., amixture of ammonium fluoride and hydrofluoric acid), may be used toremove the sacrificial spacer layer 24. After removing the sacrificialspacer layer 24, a cavity 32 is formed between the first middle spacerlayer 22 and the second middle spacer layer 26.

Referring now to FIG. 4, there is illustrated the exemplarysemiconductor structure of FIG. 3 after epitaxially growing a firstepitaxial doped semiconductor material layer 34 from exposed sidewallsurfaces of the middle portion of the semiconductor channel materialpillar 12 and within cavity 32, and a second epitaxial dopedsemiconductor material layer 36 from exposed sidewall surfaces of anupper portion of the semiconductor channel material pillar 12. The firstand second epitaxial doped semiconductor material layers 34, 36 areformed simultaneously utilizing an epitaxial growth process as mentionedabove.

The first epitaxial doped semiconductor material 34 and the secondepitaxial doped semiconductor material 36 comprise a semiconductormaterial that contains a p-type or n-type dopant. The conductivity typeof the dopant (i.e., n or p) is the same as the conductivity type of thedopant present in the bottom source/drain layer 16. The semiconductormaterial that provides the first epitaxial doped semiconductor material34 and the second epitaxial doped semiconductor material 36 may be thesame as, or different from, the semiconductor material that provides thebottom source/drain layer 16. The concentration of dopant within thesemiconductor material that provides the first epitaxial dopedsemiconductor material 34 and the second epitaxial doped semiconductormaterial 36 is within the concentration range mentioned above for thebottom source/drain layer 16. The first epitaxial doped semiconductormaterial 34 has an epitaxial relationship with the middle portion of thesemiconductor channel material pillar, while the second epitaxial dopedsemiconductor material 36 has an epitaxial relationship with the upperportion of the semiconductor channel material pillar. As is shown, thewidth of the first epitaxial doped semiconductor material 34 and thesecond epitaxial doped semiconductor material 36 is less than the widthof the remaining material layers of the combined first and secondmaterial stacks.

Referring now to FIG. 5, there is illustrated the exemplarysemiconductor structure of FIG. 4 after performing a drive in anneal toprovide a bottom source/drain region 16S, a middle source/drain region34S and a top source/drain region 36S. Notably, FIG. 5 illustrates theexemplary semiconductor structure of FIG. 4 after an anneal is performedto diffuse dopants from the bottom source/drain layer 18 into a bottomportion of the semiconductor channel material pillar 12 and to providethe bottom source/drain region 16S, dopants from the first epitaxialdoped semiconductor material layer 34 into the middle portion of thesemiconductor channel material pillar 12 and to provide a middlesource/drain region 34S, and dopants from the second epitaxial dopedsemiconductor material layer 36 into the upper portion of thesemiconductor channel material pillar 12 and to provide a topsource/drain region 36S.

The anneal (i.e., the drive in anneal) may be performed at a temperaturefrom 700° C. to 1300° C., depending on the annealing time. Typicallyhigher temperatures require less annealing times. Annealing can be doneby rapid thermal anneal (RTP), laser anneal, flash anneal, furnaceanneal, or any suitable combination of those techniques. In oneembodiment, the anneal is done at 1000° C. for 0.5 seconds. Othertemperatures may also be used as long as the anneal temperature iscapable of forming the bottom source/drain region 16S, the middlesource/drain region 34S and the top source/drain region 36S shown inFIG. 5. In some embodiments, the anneal may be performed in an inertambient such as, for example, helium and/or argon. In other embodiments,the anneal may be performed in a forming gas ambient. The duration ofthe anneal may vary so long as the duration of the anneal causes dopantdiffusion and the formation of the bottom source/drain region 16S, themiddle source/drain region 34S and the top source/drain region 36S shownin FIG. 5.

In accordance with the present application, the bottom source/drainregion 16S consists of a first doped semiconductor material (i.e., thebottom source/drain layer 16) and a doped lower portion of thesemiconductor channel material pillar 12, the middle source/drain region34S consists of a second doped semiconductor material (i.e., the firstepitaxial doped semiconductor material 34) and a doped middle portion ofthe semiconductor channel material pillar 12, and the top source/drainstructure 36S consists of a third doped semiconductor material (i.e.,the second epitaxial doped semiconductor material layer 34) and a dopedupper portion of the semiconductor channel material pillar 12. Remainingportions of the semiconductor channel material pillar 12 that are notused in forming one of the source/drain regions may be referred to as achannel portion 12P of the semiconductor channel material pillar 12.

In one embodiment, the bottom source/drain region 18S can be used as asource region of a subsequently formed lower functional gate structure,the middle source/drain region 34S can be used a shared drain region ofthe subsequently formed lower functional gate structure and asubsequently formed upper functional gate structure, and the topsource/drain region 36S can be used a source region of the subsequentlyformed upper functional gate structure.

Referring now to FIG. 6, there is illustrated the exemplarysemiconductor structure of FIG. 5 after replacing the first sacrificialgate structure 20 with a lower functional gate structure 38L, 40L, andthe second sacrificial gate structure 28 with an upper functional gatestructure (38U, 40U), and forming an interlevel dielectric (ILD)material 42. Typically, the ILD material is formed prior to replacingthe first and second functional gate structures 20, 28.

The ILD material 42 may be composed of silicon dioxide, undoped silicateglass (USG), fluorosilicate glass (FSG), borophosphosilicate glass(BPSG), a spin-on low-k dielectric layer, a chemical vapor deposition(CVD) low-k dielectric layer or any combination thereof. The term“low-k” as used throughout the present application denotes a dielectricmaterial that has a dielectric constant of less than silicon dioxide. Inanother embodiment, a self-planarizing material such as a spin-on glass(SOG) or a spin-on low-k dielectric material such as SiLK™ can be usedas ILD material 42. The use of a self-planarizing dielectric material asthe ILD material 42 may avoid the need to perform a subsequentplanarizing step.

In one embodiment, the ILD material 42 can be formed utilizing adeposition process including, for example, chemical vapor deposition(CVD), plasma enhanced chemical vapor deposition (PECVD), evaporation orspin-on coating. In some embodiments, particularly whennon-self-planarizing dielectric materials are used as the ILD material42, a planarization process or an etch back process follows thedeposition of the dielectric material that provides the ILD material 42.

The first and second sacrificial gate structures 20, 28 are typicallyremoved simultaneously utilizing an etching process that is selective inremoving the material that provides the first and second sacrificialgate structures 20, 28. In one embodiment, and when the first and secondsacrificial gate structures 20, 28 are composed of polysilicon, ammoniaor TMAH (tetramethylammonium hydroxide) can be used as an etchant. Inanother embodiment, and when the first and second sacrificial gatestructures 20, 28 are composed of amorphous silicon, ammonia or TMAH canbe used as an etchant.

The removal of the first sacrificial gate structure 20 provides a lowergate cavity (not shown) that exposes sidewall surfaces of the channelportion 12P that is located between the bottom source/drain region 18Sand the middle source/drain region 34S and between the bottom spacerlayer 18 and the first middle spacer layer 22, while the removal of thesecond sacrificial gate structure 28 exposes sidewall surfaces of thechannel portion 12P that is located between the middle source/drainregion 34S and the top source/drain region 36S and between the secondmiddle spacer layer 26 and the top spacer layer 30.

Each gate cavity is then filled with a gate dielectric portion (38L,38U) and a gate conductor portion (40L, 40U). Each gate dielectricportion (38L, 38U) may be composed of a gate dielectric material suchas, for example, an oxide, nitride, and/or oxynitride. In one example,the gate dielectric portion (38L, 38U) can be a high-k material having adielectric constant greater than silicon dioxide. Exemplary high-kdielectrics include, but are not limited to, HfO₂, ZrO₂, La₂O₃, Al₂O₃,TiO₂, SrTiO₃, LaAlO₃, Y₂O₃, HfO_(x)N_(y), ZrO_(x)N_(y), La₂O_(x)N_(y),Al₂O_(x)N_(y), TiO_(x)N_(y), SrTiO_(x)N_(y), LaAlO_(x)N_(y),Y₂O_(x)N_(y), SiON, SiN_(x), a silicate thereof, and an alloy thereof.Each value of x is independently from 0.5 to 3 and each value of y isindependently from 0 to 2. In some embodiments, a multilayered gatedielectric structure comprising different gate dielectric materials,e.g., silicon dioxide, and a high-k gate dielectric, can be formed andused as each gate dielectric portion (38L, 38U). The gate dielectricmaterial that provides the gate dielectric portion (38L, 38U) can beformed by any deposition process including, for example, chemical vapordeposition (CVD), plasma enhanced chemical vapor deposition (PECVD),physical vapor deposition (PVD), sputtering, or atomic layer deposition.In one embodiment of the present application, the gate dielectricmaterial that provides the gate dielectric portion (38L, 38U) can have athickness in a range from 1 nm to 10 nm. Other thicknesses that arelesser than, or greater than, the aforementioned thickness range canalso be employed for the gate dielectric material that provides gatedielectric portion (38L, 38U).

Each gate conductor portion (40L, 40U) includes a gate conductormaterial. The gate conductor material used in providing the gateconductor portion (40L, 40U) can include any conductive materialincluding, for example, doped polysilicon, an elemental metal (e.g.,tungsten, titanium, tantalum, aluminum, nickel, ruthenium, palladium andplatinum), an alloy of at least two elemental metals, an elemental metalnitride (e.g., tungsten nitride, aluminum nitride, and titaniumnitride), an elemental metal silicide (e.g., tungsten silicide, nickelsilicide, and titanium silicide) or multilayered combinations thereof.In one embodiment, the gate conductor material may comprise an nFET gatemetal. In another embodiment, the gate conductor material may comprise apFET gate metal. The gate conductor material used in providing the gateconductor portion (40L, 40U) can be formed utilizing a depositionprocess including, for example, chemical vapor deposition (CVD), plasmaenhanced chemical vapor deposition (PECVD), physical vapor deposition(PVD), sputtering, atomic layer deposition (ALD) or other likedeposition processes. When a metal silicide is formed, a conventionalsilicidation process is employed. In one embodiment, the gate conductorportion (40L, 40U) can have a thickness from 50 nm to 200 nm. Otherthicknesses that are lesser than, or greater than, the aforementionedthickness range can also be employed for the gate conductor portion(40L, 40U).

As is shown in FIG. 6, each gate dielectric portion (38L, 38U) has afirst portion that directly contacts a channel portion 12P of thesemiconductor channel material pillar 12, a second portion that directlycontacts a topmost surface of the gate electrode portion (40L, 40U), anda third portion that directly contacts a bottommost surface of the gateelectrode portion (40L, 40U).

Referring again to FIG. 6, there is shown the semiconductor structure ofthe present application. Notably, the semiconductor structure includes alower functional gate structure (38L, 40L) located adjacent sidewallsurfaces of a lower channel portion 12P of a semiconductor channelmaterial pillar 12. An upper functional gate structure (38U, 40U) islocated above the lower functional gate structure (38L, 40L) andadjacent sidewall surfaces of an upper channel portion 12P of thesemiconductor channel material pillar 12. A bottom source/drain region16S is located beneath the lower functional gate structure (38L, 40L), amiddle source/drain region 34S is located between the lower functionalgate structure (38L, 40L) and the upper functional gate structure (38U,40U), and a top source/drain region 36S is located above the upperfunctional gate structure (38U, 40U).

The semiconductor structure of FIG. 6 also includes the bottom spacerlayer 18 located between the lower functional gate structure (38L, 40L)and the bottom source/drain region 16S, the first middle spacer layer 22located between the lower functional gate structure (38L, 40L) and themiddle source/drain region 34S, and a second middle spacer layer 26located between the middle source/drain region 34S and the upperfunctional gate structure (38U, 40U), and the top spacer layer 30located between the upper functional gate structure (38U, 40U) and thetop source/drain region 36S. As is further shown, lower functional gatestructure (38L, 40L) and upper functional gate structure (38U, 40U),which are vertically stacked, surround channel portions 12P of thesemiconductor channel material pillar 12.

Referring now to FIG. 7, there is illustrated the exemplarysemiconductor structure of the present application after forming variouscontact structures; the ILD material is not shown for clarity. Notably,FIG. 7 illustrates the exemplary semiconductor structure of the presentapplication after forming a gate contact structure 50 contacting boththe upper functional gate structure (G2=38U,40U) and lower functionalgate structure (G1=38L,40L), a first source/drain contact structure 52contacting the top source/drain region 36S, a second source/draincontact structure 54 containing the middle source/drain region 34S, anda third source/drain contact structure 56 contacting the bottomsource/drain region 16S. Each contact structure may be formed byconventional means well known to those skilled in the art. Each contactstructure may be composed of a contact metal such as, for example,copper, tungsten or a copper-tungsten alloy.

While the present application has been particularly shown and describedwith respect to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formsand details may be made without departing from the spirit and scope ofthe present application. It is therefore intended that the presentapplication not be limited to the exact forms and details described andillustrated, but fall within the scope of the appended claims.

What is claimed is:
 1. A method of forming a semiconductor structure,the method comprising: forming a material stack adjacent sidewallsurfaces of a semiconductor channel material pillar, wherein thematerial stack comprises, from bottom to top, a bottom source/drainlayer, a bottom spacer layer, a first sacrificial gate structure, afirst middle spacer layer, a sacrificial spacer layer, a second middlespacer layer, a second sacrificial gate structure and a top spacerlayer; removing the sacrificial spacer layer located between the firstand second middle spacer layers to physically expose sidewall surfacesof a middle portion of the semiconductor channel material pillar;epitaxially growing a first epitaxial doped semiconductor material layerfrom exposed sidewall surfaces of the middle portion of thesemiconductor channel material pillar, and a second epitaxial dopedsemiconductor material layer from exposed sidewall surfaces of an upperportion of the semiconductor channel material pillar; performing ananneal to diffuse dopants from the bottom source/drain layer into abottom portion of the semiconductor channel material pillar and toprovide a bottom source/drain region, dopants from the first epitaxialdoped semiconductor material layer into the middle portion of thesemiconductor channel material pillar and to provide a middlesource/drain region, and dopants from the second epitaxial dopedsemiconductor material layer into the upper portion of the semiconductorchannel material pillar and to provide a top source/drain region; andreplacing the first sacrificial gate structure with a lower functionalgate structure, and the second sacrificial gate structure with an upperfunctional gate structure.
 2. The method of claim 1, wherein a hard maskcap is present on the semiconductor channel material pillar prior tosaid forming said material stack, and the hard mask cap is removed afterperforming the anneal.
 3. The method of claim 1, wherein the bottomsource/drain region is a source region of the lower functional gatestructure, the middle source/drain region is a shared drain region ofthe lower functional gate structure and the upper functional gatestructure, and the top source/drain region is a source region of theupper functional gate structure.
 4. The method of claim 1, wherein thesacrificial spacer layer comprises a different spacer material than thefirst and second middle spacer layers and the top spacer layer.
 5. Themethod of claim 4, wherein the sacrificial spacer layer comprisessilicon dioxide, and the first and second middle spacer layers and thetop spacer layer each comprise silicon nitride.
 6. The method of claim2, wherein the replacing the first sacrificial gate structure and thesecond sacrificial gate structure comprises: forming an interleveldielectric material on exposed surfaces of the top source/drain regionand the middle source/drain region; removing the first and secondsacrificial gate structures to provide first and second gate cavities,respectively; and forming a gate dielectric portion and a gate electrodeportion is each of the first and second gate cavities.
 7. The method ofclaim 6, wherein each gate dielectric portion has a first portion thatdirectly contacts a channel portion of the semiconductor channelmaterial pillar, a second portion that directly contacts a topmostsurface of the gate electrode portion, and a third portion that directlycontacts a bottommost surface of the gate electrode portion.
 8. Themethod of claim 2, further comprising forming a gate contact structurecontacting both the upper and lower functional gate structures, a firstsource/drain contact structure contacting the top source/drain region, asecond source/drain contact structure containing the middle source/drainregion, and a third source/drain contact structure contacting the bottomsource/drain region.
 9. The method of claim 1, wherein the semiconductorchannel material pillar is epitaxially grown from a semiconductormaterial surface of a semiconductor substrate.
 10. The method of claim9, wherein prior to epitaxially growing the semiconductor channelmaterial pillar, a sacrificial dielectric material having an opening isformed on the semiconductor substrate.
 11. The method of claim 1,wherein the semiconductor channel material pillar is formed byepitaxially growing a semiconductor channel material on a surface of asemiconductor substrate, and patterning the semiconductor channelmaterial to provide the semiconductor channel material pillar.
 12. Themethod of claim 11, further comprising forming a hard mask layer on thesurface of the semiconductor channel material prior to patterning, andwherein the patterning of the semiconductor channel material alsopatterns the hard mask layer.
 13. The method of claim 1, wherein thesemiconductor channel material pillar is composed of a semiconductormaterial having a higher electron mobility than a semiconductorsubstrate in which the semiconductor channel material is disposed on.14. The method of claim 1, wherein the bottom source/drain layercomprises a first doped semiconductor material and the bottomsource/drain layer is formed by epitaxial growth.
 15. The method ofclaim 1, wherein the removing of the sacrificial spacer layer comprisesa selective etching process.
 16. The method of claim 1, wherein thefirst and second epitaxial doped semiconductor material layers comprisean n-type dopant.
 17. The method of claim 1, wherein the first andsecond epitaxial doped semiconductor material layers comprise a p-typedopant.
 18. The method of claim 1, wherein the anneal is performed at atemperature from 700° C. to 1300° C.
 19. The method of claim 1, furthercomprising forming, after the replacing of the first and secondsacrificial gate structures, an interlevel dielectric material.
 20. Themethod of claim 1, wherein the replacing of the first and secondsacrificial gate structures are performed simultaneously utilizing aselective etching process.